The present invention relates to the mounting of a current transformer for use with a dynamoelectric machine and, more particularly, the mounting of a radio frequency current transformer to the lead box of a turbine generator.
The operation of turbine generators can adversely be affected by various types of faults. One such type of fault is caused by electrical arcing between strands, or between adjacent ends of a broken strand, in the high voltage stator winding conductors of a generator. The stator windings of a large generator are made up of stranded conductors encapsulated within relatively heavy high-voltage insulation and suitably connected together. Arcing faults may occur due to the breaking of one or more strands of the conductors themselves or the connections between conductors, or to local insulation failures between adjacent strands. These types of faults can cause intermittent or repeated arcing between the strands, or the strand ends involved, and lead to increasing overheating and eventual break-down of the major ground insulation, with resulting serious electrical failure. It is therefore desirable to detect the occurrence of such a fault at an early stage before any serious damage has occurred, so that remedial action may be taken. One such way for providing early detection of arcing faults is described in U.S. Pat. No. 4,156,846 issued to Harrold on May 29, 1979. It describes a method for detecting the presence of arcing faults in turbine generators by detecting high frequency currents which flow in the neutral lead of the generator winding when such arcing exists. The presence of this type of arcing can thus be detected by observing the high frequency neutral currents and monitoring them for the presence of the characteristic frequency distribution resulting from the resonance characteristics of the current path. In application, the radio frequency currents are monitored by connecting a high frequency current transformer to the neutral grounding lead, usually at the point where the generator's neutral grounding lead connects to the neutral transformer.
The current transformer must be properly installed on the neutral lead because, otherwise, voltage potential hazards will exist. Under normal generator operating conditions, the generator neutral lead's voltage is very close to ground potential. However, due to third harmonic current flow in the neutral lead, a voltage potential of approximately 300 VAC exists across the high voltage side of the neutral transformer. Also, during a generator fault condition, the neutral voltage can approach the generator's operating voltage and transient voltages can exceed even this value. The exact magnitude of voltage potential on the generator's neutral lead depends on a number of factors. However, it is important that the installation of the current transformer does not in any way degrade the performance of the generator's ground protection system. Since the outer case of the high frequency current transformer is at ground potential through a connecting coaxial cable and radio frequency monitor, adequate insulation must be provided to prevent electrical shorting between the generator's neutral ground protection system and the current transformer.
A present method of connecting and insulating the radio frequency current transformer consists of hand taping the neutral lead with a required minimum number of half-lapped layers of silicone rubber tape based on the generator's operating voltage. The purpose of this insulation is to prevent electrical shorts from developing between the case of the current transformer and the neutral lead. Typically, more than the recommended minimum number of layers are applied in order to build the neutral lead's diameter to that of the current transformer's inside diameter. On some neutral leads, the required minimum number of layers cannot be applied because the neutral lead's diameter is only slightly smaller than the current transformer's inside diameter. Furthermore, in some cases, the neutral lead is actually larger than the inside diameter of the current transformer and insulation in these cases requires extensive modification to the neutral grounding circuit. Furthermore, care must be taken to ensure that the current transformer is tightly attached to the neutral lead because the neutral lead is subjected to a high level of vibration and, if relative movement exists between the neutral lead and the current transformer, the insulation between the neutral lead and the current transformer may be abraded, causing the possibility of electrical shorts therebetween.